1. The Field of the Invention
The present invention relates to a light reflecting system particularly suitable for use in highway marking for reflectorizing and therefore imparting nighttime visibility to traffic regulating indicia, such as traffic lane dividing lines and the like. The light reflecting system of the invention appertains to the class of the reflex-reflecting composite elements or collinear reflectors, both such terms having been accepted and widely used in the recent patent and scientific literature for defining optical means having a face exposed to light and capable of reflecting a ray of light back to its source along its own incident path irrespective (within practical limits) of the angle of incidence.
2. The Prior Art
A reflecting system of the above class comprises at least one substantially transparent, substantially spherical element acting as a primary condensing lens, having a refractive index such that a beam of light impinging said element is refracted and at least partially focused regardless of the angle of impingement; a substantially transparent binding medium contacting and partially surrounding said element so that a portion of said element is exposed for impingement by beams of light and a portion of said element is surround by said binding medium, and reflecting means for the further focusing and the reflection of light beams which pass through said element and binding medium, said reflecting means consisting at least almost entirely of reflective substantially part-spherical surfaces which are formed or secured about the surrounded portion of said element and which are spaced from the spherical surface of said element by said binding medium.
Reflecting systems as above have been extensively discussed in my prior British Patent Specification No. 1,343,196.
For simplicity of manufacture, said reflecting systems are made as entirely globular aggregates, wherein the spherical element is completely surrounded by the binding medium and reflecting means. Such aggregates are at least partially embedded within a layer of a settable composition, painted or otherwise deposited on a surface to form retroreflecting areas, as well known in the art. When the layer composition has set, the portions of said reflecting means and binding medium, bulging from the surface of the set layer, are removed such as by brushing off or by the erasing process exerted by the vehicle traffic over said areas, so that the corresponding bulging portions of the spherical elements will become exposed for light beam impingement. The system of the invention will be herein described and compared with the prior art in its latter "service" condition, irrespective of its above "manufacture" condition.
Reflectors of the retroreflecting type are generally employed where the incident beam strikes a reflecting surface which is otherwise in partial or total darkness. In particular, retroreflecting surfaces are used on highway marking signs and advertising signs. In such cases, it is absolutely essential that the reflected light is trasmitted back to the source of the incident light, even through the incident light might strike the reflecting surface at an angle. In the case of a highway sign it is also necessary that a maximum amount of brilliancy is retained so that the sign may be visible from a suitable distance and may be clear enough to avoid any possible misinterpretation of its meaning.
This ability is of the greatest importance when the sign is formed on the roadway pavement, wherein the reflectors are impinged by "grazing" light and the retroflected light constitutes only a small fraction of the emitted light. A slight increase in such fraction might provide a sharp improvement in the "threshold distance" of visibility to the human eye. Thus, a search for an ever greater "optical efficiency" is a long felt want in the art.
Such optical efficiency (in terms of the ratio of the amount of impinging light to reflected light) can be improved both by improving the exactness of the focusing and by limiting the light losses when the light passes back and forth through the various transparent components of the system. Such losses are mainly caused by light dissipation and by the reflection, which occurs when a light ray passes through the interface between two transparent media, having different refractive indexes where the ray impinges said interface at an angle different from 90.degree..
With relation to the refractive indices of the various transparent components of the aggregate, the prior art has suggested that the spherical element generally be made of a glass the refractive index of which is between 1.46 and 1.65 and that the binding medium consist of a polymeric compound of refractive index from 1.35 to 1.50. The provision of binding medium having a refractive index noticeably below that of the spherical element contributes to improved focusing, because a better convergence of light beams is obtained at the "spherical element -- binding medium" interface. On the other hand, such convergence leads to an undesirable loss of efficiency as a result of the increased light dissipation resulting from the reflection at said interface.